An informative and technical description of the Boeing systems which may have fed erroneousinfo to the Pilot and F/O.
Boeing issues 737 Operations Manual Bulletin after Lion Air accident
"November 7, 2018, © Leeham News.: Boeing issued a message to the operators of 737 MAX aircraft yesterday to remind their pilots of the procedures if an unreliable Angle Of Attack (AOA) information is suspected while flying.
Below we describe what these procedures are and why Boeing is reminding its customers about what to do when suspecting a false AOA reading."
Boeing’s 737 AOA sensors and how it affects the aircraft
We have read the Boeing bulletin, what has been issued from the Indonesian crash investigators and the FAA, and what has been discussed in media. Below, we try to bring some clarity and perspective into what is sometimes sensational headlines out of a minimum of facts.
The Federal Aviation Administration has today issued an Airworthiness Directive for the 737-8 and -9:
“The FAA has issued an Emergency Airworthiness Directive (AD) that addresses possible erroneous angle of attack (AOA) inputs on Boeing 737 Max aircraft. These erroneous inputs can potentially make the horizontal stabilizers repeatedly pitch the nose of the airplane downward, making the aircraft difficult to control. The AD orders operators to revise the airplane flight manual (AFM) to give the flight crew horizontal stabilizer trim procedures to follow under certain conditions. The AD is effective immediately. Operators have three days to revise the AFM. The FAA continues to work closely with Boeing, and as a part of the investigative team on the Indonesia Lion Air accident, may take further appropriate actions depending on the results of the investigation. The FAA has alerted foreign airworthiness authorities who oversee operators that use the 737 MAX of the agency’s action. “.
The Flight Crew Operations Manual (FCOM) bulletin from Boeing reminds operators that the existing procedures are the correct actions to be taken if an aircraft encounters a false stall warning and flight control recovery triggered by a faulty Angle Of Attack signal.
These procedures and the sensor system triggering them has stayed the same since 20 years as far as we understand. The system and its sensors have not been changed since the introduction of the 737NG in 1998.
In the case of the Lion Air Flight JT610 the correct identification and action upon an incorrect AOA sensor information seem not to have been made. This might have been made more difficult by a simultaneous false reading of airspeed from the same system (one of three systems generating airspeed on the aircraft)."
Continues with diagrams and systems here:
https://leehamnews.com/2018/11/07/bo...-air-accident/
It finishes with the following:
"One can react to the above actions of the flight control system in the 737. But one should know this chain of actions have been defined by the 737 Flight Control team after flying the 737 since 1967, with many thousands of work hours research behind the design and over 50 years of airliner flight control design experience. It has also operated without known adverse effects in thousands of 737NG over the last 20 years.
In this case, the events might have overloaded the Flight Crew on JT610. We don’t know why. And it’s best to wait until we do. We need to understand what exactly the flight crew was confronted with and their actions as a result.
It’s by now clear it’s not a simple fact of them stalling the aircraft due to one side airspeed readings being unreliable. They had more to fight, so far is understood. But exactly what, we don’t know and therefore we don’t speculate further.
What we wanted to do with this article is pointing out, as far as we know, the system involved is a tried and tested system. Why this didn’t help the JT610 crew, we don’t know, but it will be communicated in due course."
The comments are generally supportive, some suggests differences in the MAX version, others suggest if it was an Airbus plane the FAA would be more demanding in terms of "actions to be taken". But that happens in many of the articles.